Discovering Hadean Earth

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Intellectual merit. The first 500 million years of Earth history - the Hadean Eon - is a period of 'lost' time as there is virtually no rock record preserved from this interval. However, the PI and colleagues have samples of detrital and xenocrystic zircons from western Australia (Jack Hills) with ages between 4.0 and 4.4 Ga. A massive analysis campaign has identified many thousands of these ancient zircons, and investigations of these grains have been interpreted to suggest the presence of liquid water at the Earth's surface, significant growth of continental crust, a mature sediment recycling system, and perhaps operation of plate boundary-type processes during the Hadean. Such evidence suggests the possibility that the very early planet was much more similar to the present day Earth than previously thought and this stimulates and justifies new measurements. Analytical developments, analysis programs, and experiments are proposed to address the following questions: 1) Could Hadean Jack Hills zircons have been formed by impacts? 2) Do Hadean zircons record the terrestrial counterpart to the ~3.9 Ga lunar cataclysm? 3) What happened to the highly enriched (low Lu/Hf) reservoir seen in 3.9- 4.3 Ga zircons? 4) Do mineral inclusions in >4 Ga zircons retain a pristine Hadean record? 5) Are available barometers applicable to inclusions in Hadean zircons? 6) Do Pu-U variations in Hadean zircons reflect water-mediated fractionation? and 7) Are the collective isotopic, mineralogical, and thermometry data evidence of Hadean continental recycling and plate boundary interactions? Proposed investigations include: 1) Laser ablation ICPMS measurement of initial 176Hf/177Hf in 3.9-3.6 Ga zircons, 2) U-Th-Pb ion microprobe depth profiling of >4 Ga zircons, and 3) characterization of zircons formed in terrestrial impact melts, 4) Hadean Pu-U geochemistry from fission Xe isotopes, 5) Nd isotopic systematics of Jack Hills zircon, 6) NanoSIMS analysis of Si/Al in micron-sized muscovite, 7) K-Ca dating of Hadean muscovite inclusions, and 8) calibration of the Si-in-muscovite barometer for peraluminous granitoids. Broader Impacts. This research will directly benefit the UCLA graduate students supported by this project. Once trained on the ion microprobe, these students in turn provide support to visitors (including numerous graduate students) to the NSF-supported Keck Center for Isotope Geochemistry (see below). Several small projects have been identified that will be used to engender undergraduate research opportunities. Research outcomes are of international interest and could be akin to the results of a planetary exploration mission if the Hadean paradigm is overturned. This project showcases scientific and technical leadership in a field of global interest, aids in capturing the imagination of young people considering a career in science and technology, and provides training opportunities for young scientists. Research results will be communicated to the scientific community via high citation impact scientific journals and to the public through sustained outreach activities (see http://sims.ess.ucla.edu/outreach/outreach.php).